One type of data storage apparatus is a track-following disk file which has a stack of data recording disks mounted for rotation on a central spindle. A plurality of recording heads are moved in unison in relation to respective disk surfaces to, for example, write data on concentric data tracks. Another servo head is moved in unison with the recording heads in relation to another disk surface which stores on concentric tracks a servo data encoding scheme having timing information and position-indicating-information interposed between the timing information.
A servo system of the disk file typically has a phase-lock oscillator for generating various control signals. The phase-lock oscillator responds to the timing information transduced by the servo head to generate a write clocking frequency signal used to write data on the data tracks with the recording heads. The phase-lock oscillator also responds to the timing information to generate gating signals which are "windows" for detecting and demodulating the position-indicating-information read by the servo head to center the recording heads in relation to the data tracks.
Generally, a phase-lock oscillator has a voltage controlled oscillator (VCO) which generates the write clocking frequency signal and a phase detector which generates an error signal used to control the VCO. The phase detector produces the error signal in response to any phase difference between the edges of two input signals. The phase-lock oscillator has a control circuit which generates the two input signals, one being derived from the frequency signal generated by the VCO and the other being derived from the timing information of the servo encoding scheme. When, for example, an edge of one input signal is in phase with the corresponding edge of the other input signal, the frequency signal is accurately synchronized with the timing information. Consequently, data can be written at accurate positions on the data tracks and properly timed gating signals can be generated from the synchronized frequency signal to detect and demodulate the position-indicating-information. In particular, and as an example, the control circuit can include a long-time-constant monostable element which responds to the timing information by generating a pulse, i.e., the one input signal, whose trailing edge is compared to the rising edge of the other input signal. For example, this pulse may have a duration of 600 nanoseconds.
One problem with certain prior implementations of phase-lock oscillators is that a long-time-constant monostable element of the control circuit has the disadvantges of noise sensitivity, temperature drift, and ill-defined initial delay. Noise sensitivity means that the monostable element can be triggered by circuit or line noise, temperature drift means that the pulse duration can vary in dependence on temperature, and ill-defined initial delay means that the generation of the pulse can occur at varying delays, all of which affect the timing or phase of the above-mentioned trailing edge. While control circuits employing such a long-time-constant monostable element can be pre-adjusted to overcome these component tolerances such as drift and initial delay, this adjustment itself is undesireable and imprecise. Furthermore, after adjustment, the control circuit can still exhibit unwanted sensitivity to power-supply variation and noise. Also, some control circuits do use relatively short-time-constant monostable elements which reduce the problems of drift and initial delay, but these are highly complex timing components which require pre-adjustment.
Also, in prior phase-lock oscillators, a relatively long duration "window" is provided during which the phase-lock oscillator is waiting for the timing information as the disk servo surface rotates. Defects manifesting themselves as noise pulses can appear anywhere on the servo surface. Consequently, the longer the duration of the "window", the greater the likelihood that a defect will appear to falsely trigger the control circuit.
Other implementations of phase-lock oscillators are predominantly analogue. These do not have the advantages of a substantially digital construction that generally exists for digital circuits over analog circuits.
In addition, detection of loss of synchronization to the servo timing information is essential since, upon loss, the gating signals cannot be generated at accurate times to detect and demodulate the position-indicating-information. Prior servo systems employ special circuits auxiliary to the phase-lock oscillator which, therefore, add significantly to the cost and complexity of the overall servo system.